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Expression engine sandbox escape via DotList map/filter and fallback resolver leads to authenticated RCE

High
mbakgun published GHSA-87x2-9jwx-7gh4 Aug 15, 2026

Package

heym

Affected versions

<= 0.0.90

Patched versions

0.0.91

Description

Summary

The expression engine's simpleeval hardening from GHSA-pm6h-x3h5-j38h covers the condition/substitution paths, but two sibling paths never pass through that guard and walk dunder attributes with raw getattr:

  1. DotList item expressions used by .map() / .filter() / distinctBy() (backend/app/services/workflow_executor.py, _evaluate_item_expr ~L898-915 and the _ITEM_DOT_PATH_RE fast path in _evaluate_item_expression ~L950-980). The regex ^item(?:\.[a-zA-Z_][a-zA-Z0-9_]*)+$ matches item.__class__.__base__.__subclasses__, and each segment is resolved with hasattr/getattr with no dunder block.
  2. The _resolve_simple_expression fallback (~L6325-6450) that runs whenever HeymExpressionEval.eval raises (which is exactly what simpleeval does on dunder access). It resolves method calls and properties with raw getattr and calls callables, and _read_property_with_subscripts (~L6853) resolves further dunder properties and subscripts.

Together these restore the full __subclasses__ -> __globals__ -> os.system gadget that GHSA-pm6h was meant to close.

Impact

Any authenticated user who can edit and run a workflow (set node, output expression, JSON mapper, variable nodes, or the /api/expressions/evaluate endpoint) executes arbitrary Python as the backend process: database contents, stored credentials (Fernet key is in process env), and any reachable internal network.

Reproduction (verified at a7919b1)

Against the real WorkflowExecutor (constructed with nodes=[], edges=[]), three expressions equivalent to three set-node values in one run:

1. $arr.map("item.__class__.__base__.__subclasses__")[0]     with arr = ["x"]
   -> returns the bound method object (type builtin_function_or_method)

2. $m()                                                        with m = result of 1
   -> list of all loaded classes (31 in a stock backend process)

3. $classes[16].__init__.__globals__["os"].system("touch /tmp/heym_pwned")
   -> returns 0; file created. Index varies by environment; any class whose
      __init__.__globals__ contains "os" works (configparser._Line did here).

Step 3 is rejected by simpleeval (dunder attribute) and then silently evaluated by the _resolve_simple_expression fallback, which is the crux: the fallback turns the sandbox's rejection into a different, unguarded evaluator.

Suggested remediation

  1. Reject any path/method segment starting with _ in _evaluate_item_expr, the _ITEM_DOT_PATH_RE fast path, _resolve_simple_expression, and _read_property_with_subscripts.
  2. Make the fallback fail-closed for anything that is not a plain name/index lookup: when HeymExpressionEval.eval raises on a dunder or call it rejected, return None instead of re-evaluating with raw getattr.
  3. Route .map() / .filter() item expressions through the same simpleeval sandbox as conditions instead of the raw-getattr fast path.
  4. Add regression tests mirroring the three expressions above.

Happy to collaborate on the patch in this advisory's private fork.

Heym Team Notes

From the Heym team: this is fixed. The reported chain and every variant we
could build from it are blocked, the guarded resolvers are covered by
regression tests across the executor, the evaluator service and the API, and
we added a callable allowlist on top so the preview can no longer invoke
inherited builtins. Thanks to @SashaMIT for a clear report, a working proof of
concept, and a patch that landed with tests already attached.

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

CVE ID

No known CVE

Weaknesses

Improper Control of Generation of Code ('Code Injection')

The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment. Learn more on MITRE.

Protection Mechanism Failure

The product does not use or incorrectly uses a protection mechanism that provides sufficient defense against directed attacks against the product. Learn more on MITRE.

Credits